Support more inner comments

[JLHwung]

Q	A
Fixed Issues?	Babel-generator does not print inner comments in certain situations
Patch: Bug Fix?	Y
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This PR supports inner comments attached to generators, async methods and async do expressions. Because such inner comments must be block comments due to the noLineTerminator restrictions, we then ensure that such restrictions are respected by the babel-generator.

This PR also relaxes the condition when Babel prints single arrow binding x => {}. Before this PR we prints (x) => {} when x has attached comments. In this PR we relaxes the condition so that we can still output /* leading */x/* trailing */ => {}.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/53221/

[nicolo-ribaudo]

Can you add this test? (I don't think we can properly print it and that's fine)

class A {
  async /* 1 */ * /* 2 */ method /* 3 */ ( /* 4 */ ) /* 5 */ {}
}

[JLHwung]

Yeah. When an AST node has more than one empty spot, the inner comments are not sufficient for recreating code exactly from AST. But at least the printer can preserve such comments instead of discarding them silently.

[liuxingbaoyu]

Can this be postponed for a while? I'm going to rebase this after the #14979 merge, it's a bit cumbersome to rebase that one. Thanks!

[nicolo-ribaudo]

This is not a valid Flow comment. If the arrow function has a single parameter with a trailing block comment that starts with :, we should still print the parentheses.

[JLHwung]

Sad that flow does not support that. In that case we can partially revert the arrow function commit.

[liuxingbaoyu]

There doesn't seem to be a change here?

[JLHwung]

Ah I forgot to update the plugin test fixtures.